Recovery of AlF3 from spent potliner

ABSTRACT

A method of recovering AlF 3  from spent potliner using an acid digest to form gaseous HF which is converted to hydrofluoric acid and reacted with alumina trihydrate to form AlF 3 .

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. Ser. No. 08/977,435, filed Nov.24, 1997 now U.S. Pat. No. 5,955,042 which is a continuation-in-part ofU.S. Ser. No. 08/569,271, filed Dec. 8, 1995 now U.S. Pat. No. 5,723,097entitled “Method of Treating Spent Potliner Material from AluminumReduction Cells”.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to processes for treating spentpotliner material from aluminum reduction cells in a manner in whichhazardous wastes are converted and recycled to useful, non-hazardoussubstances. More specifically, the present invention relates to aprocess of recovering, from spent aluminum potliner material, aluminumfluoride, reusable salts such as sodium sulfate and refractory materialsuch as calcium feldspar which can be used to make brick products, forexample. Further, large amounts of energy can be recovered from thecarbon, e.g., 8000 to 9000 BTU/lb of carbon.

2. Description of the Prior Art

The Hall-Heroult process for the production of metallic aluminum datesfrom the 19^(th) Century. Many refinements to the process have beenmade, but the basic Soderberg or pre-bake configurations usingHall-Heroult cells remain the most common processes for aluminumproduction throughout the world. In these processes, the bottom andinternal walls of a cathode of an aluminum pot are formed with a linerof carbon blocks joined by conductive carbonaceous binder and wrappedwith refractory firebricks and insulating bricks, the resultingcombination being referred to as “potliner”. The insulating bricks andfirebricks are composed of material such as silica and alumina (aluminumoxide).

During the production of aluminum, the aluminum reduction pot is filledwith a bath of alumina and molten salts. Over the three to seven yearlife span of an aluminum reduction pot bath, salts migrate into thepotliner, thereby resulting in the deterioration and eventual failure ofthe utility of the aluminum cell as a cathode. During its life span, acathodic potliner may absorb its own weight in bath salt materials. Thefailed potliner material is referred to as spent potliner or SPL.

When an aluminum reduction cell is taken out of service, the SPL iscooled and fractured to facilitate subsequent handling and disposal. Thefractured SPL is a non-homogenous material which contains carbon, silicaand/or alumina from the insulating brick and firebricks, aluminum,significant quantities of sodium salts, aluminum salts and oxides,fluoride salts and traces of cyanides. On the average, a large aluminumsmelter with a production capacity of 175,000 tons of aluminum per yearwill produce about 6,000-12,000 tons of SPL per year. The quantity ofSPL generated annually in the United States alone has in recent yearsexceeded approximately 230,000 tons per year.

Because of its cyanide content, its high concentration of leachablefluoride compounds, and the high volumes of SPL produced, SPL presents asignificant environmental hazard and a major burden for aluminumproducers, who remain ultimately liable for the proper disposal of SPL.The SPL has long been listed as a hazardous waste by the U.S. federaland state environmental authorities. Current regulations require thatSPL ultimately be treated to explicitly remove the toxic cyanide, highconcentration of leachable fluoride compounds, and other characteristicswhich cause it to be listed as hazardous before it can be placed in alandfill disposal site.

Many different approaches have been tried over the years to convert SPLto non-hazardous materials. One major technique includes combustion orincineration of the SPL as exemplified in U.S. Pat. Nos. 4,735,784;4,927,459; 5,024,822; 5,164,174; 5,222,448 and 5,286,274. Unfortunately,most of these processes result in an end product consisting of a glassyslag material which still contains some hazardous, allegedlynon-leachable, materials.

Another process includes chemical treatment to convert SPL tonon-hazardous materials. In these types of processes, as exemplified byU.S. Pat. No. 4,113,831, the initial SPL constituents are replaced withcompounds which are less toxic but which compounds are still above thehazardous listing levels established by various environmentalauthorities. Moreover, these residues generally have a final volumewhich is comparable to the volume of the input.

Another major technique of converting SPL to non-hazardous materialsincludes pyrohydrolysis of, the SPL material. This process generallyincludes pyrolysis of the material in conjunction with the introductionof water to create an off-gas containing the fluoride materials asillustrated in U.S. Pat. No. 4,113,832. Such pyrohydrolysis techniquesmay also be used in conjunction with fluidized bed reactors as disclosedin U.S. Pat. Nos. 4,158,701 and 4,160,808. These processes also stilltend to produce large volumes of waste material which must be stored inlandfills and which may contain allegedly non-leachable hazardous waste.Thus, there is still a need for a process to chemically treat SPLmaterial from aluminum reduction cells, wherein the end products of sucha treatment process are all usable either within the process itself orwith other commercial processes as well as secondary end products whichare non-toxic to the environment and which do not include large volumesof material for the landfill or for storage.

SUMMARY OF THE INVENTION

It is, accordingly, one object of the present invention to provide aprocess for treating spent potliner material from aluminum reductioncells.

It is another object of the present invention to provide such a processwherein aluminum fluoride, sodium compounds such as sodium sulfate,calcium compounds and iron compounds and refractory materials such asmullite which can be converted to brick or used as fuel or cementadditive, are all recovered from the spent aluminum potliner material ina form which is commercially usable.

Still, it is another object of the present invention to provide aprocess for treating SPL to selectively recover usable compounds such asaluminum fluoride, sodium sulfate, chloride salts, mullite and otheruseful materials therefrom.

Yet another object of the present invention is to provide a process forthe treating of spent potliner material from aluminum reduction cellswhich includes a total recycle of all by-products and elimination of allhazardous wastes.

To achieve the foregoing and other objects and in accordance with thepurpose of the present invention, as embodied and broadly describedherein, a process of treating spent potliner material from aluminumreduction cells and recovering useful products is disclosed. In theprocess of the present invention, spent potliner material is introducedinto an acid digester containing, for example, sulfuric acid. As aresult of this step, a gas component is produced which includes hydrogenfluoride and hydrogen cyanide. Also, a slurry component is producedwhich includes carbon, silica, alumina, sodium compounds such as sodiumsulfate, aluminum compounds such as aluminum sulfate, iron compoundssuch as iron sulfate, magnesium and calcium compounds such as magnesiumand calcium sulfate. The slurry component remains in the digester afterthe gas component is removed. The gas component is recovered and heatedan effective amount to convert or decompose the hydrogen cyanide to aremaining gas component including CO₂, H₂O, and nitrogen oxides, as wellas HF gas. The remaining gas component is directed through a waterscrubber in which the HF gas is converted to liquid hydrofluoric acid.The hydrofluoric acid is then admixed with alumina trihydrate to formaluminum fluoride (a commercially useful end product) and water.

The slurry component is rinsed with water to separate a solid fractioncontaining carbon, and refractory materials such as alumina and silicafrom a liquid fraction. The solid fraction may be admixed with analumina/silica mixture and then used as fuel in cement or glassmanufacturing. Alternatively, the solid fraction can then be subjectedto an elevated temperature in an oxygen-rich atmosphere. This causes thecarbon to oxidize to carbon dioxide which itself has utility as a fuel,leaving a refractory material such as mullite formed from silica andalumina which has commercial utility in forming brick.

In one aspect of the invention, the remaining liquid portion of theslurry is mixed with alcohol at a preferred ratio of about four partsalcohol to about one part liquid. This step removes in excess of 97% ofthe salts and leaves a solution of sulfonic acid and alcohol. Thissolution is then subjected to distillation, with the volatile alcoholbeing recovered for reuse, and the remaining sulfic acid available to beadded back to the system digester to reduce acid consumption. Thefiltered salts are then dissolved back in H₂O and the pH adjusted to abasic pH, e.g., about 12.0 to 12.5, with NaOH. This step holds aluminumin solution as sodium aluminate and precipitates all other impurities.The solution is filtered to remove the impurities containing calcium,iron, magnesium and silicates primarily. The clear solution is thenfurther pH adjusted to an alkaline pH, e.g., about 7.0 to 8.0 pH, toremove Al(OH)₃, and the remaining solution is then admixed with alcoholto form and precipitate sodium sulfate.

In another aspect of the invention, the remaining liquid portion of theslurry may be treated to form soluble sodium aluminate by adjusting thepH, for example, of the liquid portion. Adjusting the pH causesinsoluble salts such as calcium, iron and magnesium salts to form aprecipitate which is removed leaving a solution containing solublesodium aluminate. The insoluble salts are then filtered and reused. Theinsoluble salts are further processed using acid and heat to form a highpurity calcium compound. Also, iron compounds are precipitated andrecovered from the remaining liquid portion. In addition, magnesiumsalts are also precipitated and recovered from the remaining liquidportion. The solution remaining after calcium, iron and magnesium saltsare removed is added to the solution containing soluble sodiumaluminate. The pH of this solution is adjusted to form aluminatrihydrate which can be removed from the solution. The solutionremaining may be treated to remove residual Al(OH₃) before being addedback to the digestion step.

These and other objects of the present invention will become apparent tothose skilled in the art from the following detailed description,showing the contemplated novel construction, combination, and elementsas herein described, and more particularly defined by the appendedclaims, it being understood that changes in the precise embodiments tothe herein disclosed invention are meant to be included as coming withinthe scope of the claims, except insofar as they may be precluded by theprior art.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings which are incorporated in and form a part ofthe specification illustrate complete preferred embodiments of thepresent invention according to the best modes presently devised for thepractical application of the principles thereof, and in which:

FIG. 1 is a flow diagram illustrating the various process steps andby-products of the present invention.

FIG. 2 is a flow diagram illustrating steps in recovering AlF₃ fromspent potliner.

FIG. 3 is a flow diagram illustrating steps in recovering mullite,(NH₄)₂SO₄ and CaCl₂ from spent potliner.

FIG. 4 is a flow diagram illustrating steps in recovering Al(OH)₃,Na₂SO₄ and metal chlorides from spent potliner.

FIG. 5 is a flow diagram showing an alternate method of treatingcalcium, iron and magnesium compounds in spent potliner.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The process of the present invention for the treatment of spent potliner(SPL) waste materials is shown diagrammatically by FIG. 1, which processis generally identified by the reference numeral 10. The input material12 consists of SPL as its major ingredient, but may also include anyother waste stream with similar chemical make-up. One preferredoperation is described below, although it will be apparent to oneskilled in the art that many of the steps are optional.

In preferred operations, input material 12 is pulverized by a crusher 14to a particulate feed size of 16 mesh or less, although larger particlesmay be used. One preferred form of crusher operation is a two-stageprocess in which an initial crusher hopper 14 reduces the SPL materialto approximately two-inch size pieces, with the resulting two-inch sizepieces then being sent to a second crusher 15 which reduces them toabout 16 mesh or less in size. The particulate material from thecrushers 14 and 15 is then sent to a magnetic separator 16 which removesiron and any other ferromagnetic particulate metal 17, and in particulariron, from the particulate feed. A 16 mesh classifier 18 returns anyparticulate material which is greater than 16 mesh to crusher 14 througha return loop 19 in order to reduce the size of that material to 16 meshor less, since particulate material larger than 16 mesh is notrecommended or preferred.

The resulting particulate feed 20 may be directed initially into a soaktank 22 for a sufficient time, e.g., about 24 hours, and temperature toremove gases such as ammonia, acetylene and methane gases, the soak tankpreferably containing neutral H₂O and waste water from caustic scrubber58 used in the polishing step. The feed 23 is then directed into an aciddigester 24 containing preferably, sulfuric acid; however, other acidswhich liberate HF or HCN gases may be used singly or in combination withsulfuric acid. Particulate feed 23 is preferably fed into digester 24 bya sealed, variable drive, heated screw. The auger digester 24 ispreferably maintained under a negative pressure in order to assist inremoving gases which are generated within digester 24. In preferredoperations, the digester 24 is maintained at an elevated temperature,for example, up to 300° C. and typically 100° or 135° to 250° C. Thespeed of the preferred input and output augers are adjusted to allow foran approximately 30-60 minute retention time of the particulate feedmaterial 23 within the digester 24 with longer times not found to bedetrimental. Shorter times can be used at higher temperatures. In thedigester 24, the SPL and other materials react with the acid, e.g.,sulfuric acid, causing any fluoride and cyanide material to be convertedto HF and HCN gas, repectively, which is continuously removed from thedigester 24 in a gas stream or gas component 28. The remaining solidmaterial is removed from digester as a solid component 30.

In preferred operations, concentrated or strong acid 32, e.g., sulfuric(approximately 93% by weight) is added to the digester 24 at a rate ofapproximately 0.8 lbs H₂SO₄ to one pound particulate material, dependingto some extent on the soluble portion of the spent potlining. The ratioof acid to particulate material by weight can range from 0.8 to 1.1 forH₂SO₄ acid. While, as noted, H₂SO₄ is the preferred acid for thedigester 24, it should be understood that other acids such as HClO₄,HCl, HNO_(3,) H₃(PO₄) and oleum, or combinations thereof, may also beutilized. The different acids may produce different effluent salts.However, the process can be adjusted to accommodate the differentmaterials. Water is continuously added to the soak tank 22 at a rate tomaintain the soak tank level and to maintain approximately 20% moisturecontent within the digester 24. The moisture or liquid content in thedigester can range from 5 to 100 wt. % water to spent potliner. Inpreferred operations, the water added to the digester is purge water 36from caustic scrubber 58, as described in greater detail below. By thusrecycling the purge water, any fluoride salts captured from other partsof the process are recovered, and the water thus provided is at atemperature in the range of from about ambient to 120° F., therebysaving heating energy. The use of purge water also eliminates the needto dispose of the waste stream from caustic scrubber 58. Also, therecycling of the purge water provides for more favorable economics inthe process.

The gas component 26 from the soak tank 22 and the gas component 28leaving the digester 24 will normally contain hydrogen cyanide (HCN) andhydrogen fluoride (HF). The gas components 26 and 28 are then heated atheater or oxidizer 38. In preferred operations, heater or oxidizer 38 isin the form of an art known as electric converter/oxidizer which isdesigned to heat the gas component 28 to a temperature sufficiently highto oxidize the hydrogen cyanide, for example, to approximately 750°-850°C. in the presence of air. At this temperature, hydrogen cyanide isoxidized and converted or decomposed into a residual gas component 40including H₂O, CO₂ and NO₂, NO_(x) or N₂, while the HF gas remainsunreacted. The residual gas component 40 is then preferably cooled inwaste heat recovery boiler 42. Typically, the temperature of the gasesis reduced to less than 150°-200° C. The cooled residual gas componentis then directed into a water scrubber 44. The heat recovered in theboiler 42 is redirected to other stages of the process 10, as desired,to thereby save energy and enhance the efficiency of the process.

In the water scrubber 44, hydrogen fluoride in the residual gascomponent 40 is converted to liquid hydrofluoric acid 46 which isdirected to an alumina trihydrate reaction tank 48 in which it reactswith the alumina trihydrate to form aluminum fluoride and water. Aluminatrihydrate 136 is introduced into the reaction tank 48 from anotherportion of the process as described below. Alumina trihydrate as usedherein is meant to include Al₂O₃.3H₂O or Al(OH)₃ and may be referred toas aluminum hydroxide, aluminum hydrate, hydrated alumina or hydratedaluminum oxide. The reaction tank 48 is heated to a temperature toeffect reaction between hydrofluoric acid and the aluminum hydroxide toform aluminum fluoride. Preferably, the temperature is in the range of135° to 250° C. with a typical temperature being about 200° F. for aboutthree hours. The aluminum fluoride is then filtered at 49 and directedto a dryer 50 where the residual solids are heated to less than 10%moisture. These dried solids are then directed to a calciner or dryer 51where the solids are flash heated to a temperature of about 700° C.forming aluminum fluoride 52. Water vapor 53 is redirected from thedryer 50 and reaction calciner or dryer 51 back to the water scrubber44, thereby eliminating a waste stream at this point of the process.Gases 56 from the water scrubber 44, from which HF has been removed arethen passed to a caustic scrubber 58 as a polishing step before releaseto the atmosphere 60. In preferred form, the caustic scrubber 58utilizes NaOH to reach an alkaline or basic, e.g., a preferred, pH inthe range of about 7.0 to 8.0. In broader aspects, it will be understoodthat the pH can range from 6.5 to 10. Other alkali or alkaline earthmetal hydroxides may be used such as KOH and Ca(OH)₂, or combinationsthereof. Sodium hydroxide is preferred because it causes lesscomplications in other liquid streams of the over-all process. Asdescribed above, purge water 36 from the caustic scrubber 58 isredirected back to soak tank 22 for use therein. This eliminates anotherwaste stream in the overall process and also recaptures any residualfluorides which were unreacted with the water scrubber 58.

The aluminum fluoride 52 which is thus produced, is the first primarysolid end product of the process 10 of the present invention, and may beutilized commercially in any number of applications. For example, thealuminum fluoride 52 may be used as a bath additive for bath ratiocorrections in the cell. This substantially eliminates any environmentalproblems caused by the fluoride materials in the SPL, and, as detailedabove, provides a substantial cost benefit and savings.

Now returning to the process of the present invention at digester 24,the solid component 30 from the digester 24 is directed to a first rinsehousing 62 which receives input water 64, and thence through filter 63to a second rinse housing 66 with additional input water 65. The firstrinse 62 removes water soluble salts from the input slurry 30. In thepreferred process, the slurry 68 from the first rinse housing 62 passesthrough the filter press 63, and then the solids 69 are introduced tothe second stage water rinse housing 66 for polishing. The solid streamor fraction 70 from the second water rinse 66 includes carbon andrefractory materials such as alumina, silica, and, generally, arelatively high concentration of calcium sulfate salt. Due to this highconcentration of calcium sulfate level, the solid stream 70 passesthrough a filter 71 and into a third rinse 72 which is used in thepreferred processes to remove the soluble calcium sulfate salts from thesolids. In preferred operations wherein mullite is a desired endproduct, ammonium chloride is reacted with the calcium sulfate to formammonium sulfate and calcium chloride as indicated by the reactionformula

CaSO₄+2NH₄Cl→(NH₄)₂SO₄+CaCl₂

The aluminum chloride may be introduced as a solution 74 atapproximately 20 wt. % and introduced with rinse 72. It will beappreciated that other concentrations may be used, e.g., from 15 to 50wt. % NH₄Cl. The solution containing these two remaining salts (ammoniumsulfate and calcium chloride) are filtered at 75 and carried by stream76 to a storage unit 78 wherein they may later be recovered or reused asa calcium chloride liquid and an ammonium sulfate solid. Regardless oftheir later use, both of these salts are non- toxic and present nosubstantial environmental problem.

The solids 80 which remain after the rinses 62, 66 and 72 are filteredat 75 and are preferably directed to a mixer dryer 82 and includealumina, silica and carbon. In the alternative, the solids 80 may bedirected to a storage unit 83 wherein they may be sold and readily usedin cement manufacture or in the glass and ceramics industry. In anotherexample, to the alumina 84 and silica 85 mix at mixer dryer 82, may beadded alumina and/or silica to provide a ratio within mixer dryer 82 ata ratio of about 70% to 30%, by weight, alumina to silica, respectively.The alumina to silica ratio in solids 80 may be adjusted by the additionof alumina and/or silica. The alumina to silica ratio may be adjusted byadding alumina and/or silica to provide 40 to 90 wt. % alumina, theremainder silica on an alumina and silica basis. This alumina to silicamix 86 is then passed into a high temperature vessel 88 in which it issubjected to an elevated temperature to oxidize carbon in the mix.Typically, the temperature is in the range of about 1,600° to about2,000° C. in an oxygen-rich atmosphere. This causes any carbon remainingtherein to be oxidized to carbon dioxide, while simultaneouslyvitrifying the alumina and silica into a fused composition of aluminaand silica. Typical of the fused composition is mullite 90 which can beof high purity. Mullite 90 is a second major solid end product of theprocess of the present invention. The mullite may be utilized to makefurnace brick for use within aluminum reduction cells or for use forother commercial purposes.

In the preferred method, solids 86 are transferred to a high temperaturevessel 88 and subjected to an elevated temperature in the presence of anoxygen-rich atmosphere. This causes remaining carbon to oxidize tocarbon dioxide thereby providing 8000 to 9000 BTU/lb energy and a usablerefractory material 90, e.g., mullite.

In preferred processes, the oxygen-rich atmosphere within the vessel 88is maintained by introducing oxygen, preferably in the form of air 92,to the vessel 88. Carbon dioxide and heat as well as small amounts ofgases, HF and particulates, are removed from the vessel 88 in the formof a heated gas stream 93 and are then directed through a heat recoveryboiler 94 to a bag house 95. In the bag house 95, the particulates areremoved and redirected as bag house catch 96 to the soak tank 22, whilethe gases 97 are directed to the caustic scrubber 58 and then back tothe soak tank 22. Thus, the carbon in the SPL is used for usefulpurposes within the process 10 of the present invention as a fuel sourceto lower energy costs of the system, rather than remaining as a uselesslandfill material typical of prior SPL treatment processes or systems.

The liquid fraction 98 form the first and second rinse housings 62 and66, respectively, having been filtered at 63 is then directed to analcohol separator 100. In the separator 100, alcohol, for examplemethanol or ethanol 102, is admixed with the liquid 98 in a volume ratioof approximately 4:1 alcohol to liquid fraction, for example. The ratioof alcohol to liquid fraction can range from 10:1 to 1:5, for example,depending on the liquid fraction. This step is capable of separatingabout 97% or more of the salts in the liquid fraction 98 which arefiltered out of slurry stream 103 at filter 104. The liquid stream 106from the filter 104 includes the alcohol and excess acid from thedigester 24 and is directed through a recovery evaporation still 108wherein alcohol is separated and returned to the alcohol storage source102. The remaining sulfuric acid is stored at 110 and eventuallyreturned along line 22 a to soak tank 22 (FIG. 1) for reuse in thedigester 24. In this manner, the use of sulfuric acid and sodiumhydroxide in the process 10 can be reduced, while alcohol is recoveredand reused, thus enhancing the economics of the process 10 as comparedto prior art systems.

The salts 112 from the filter 104 are redissolved in a water bath 114and then pH adjusted in tank 116 to a basic pH, for example, preferablyusing sodium hydroxide 118 to a pH of about 12.0 to 12.5. It will beappreciated that any basic pH can be used that is effective in forming asoluble aluminate, e.g., sodium aluminate and insoluble impurities suchas metal hydroxides. For example, the pH can range from 11.8 to 13. A pHof 12 to 12.5 is an example of a pH which is effective. Also, sodiumhydroxide is an example of a metal hydroxide which can be used. However,any alkali or alkaline earth metal hydroxide may be used and iseffective in forming a soluble aluminate and insoluble metal hydroxides.For example, KOH and Ca(OH)₂ may be used. Thus, this step forms a slurry120 containing soluble sodium aluminate and insoluble impuritiesincluding calcium, iron and magnesium compounds such as calciumhydroxide, iron hydroxide and magnesium hydroxide. The insolubleimpurities are filtered at 122 and directed via solids stream 124 to thestorage tank 78.

HCl 125 can be introduced to the tank 78 to react with the metalhydroxides and produce metal chlorides, for example, to produce amixture 127 of calcium chloride, iron chloride and magnesium chloride,which mixture 127 is a useful product for use in industrial watertreatment.

The liquid fraction 126 from the filter 122 is directed to a second pHcorrection tank 128 wherein an acid 130, such as sulfuric acid, is addedto lower the pH, for example, to about 7.0 to 8.0 to precipitate aluminatrihydrate. This step forms a slurry 132 containing soluble sodiumsulfate and alumina trihydrate precipitate. It will be understood thatother acids may be used to lower the pH. Further, the pH used is a pHwhich enables separation of the sulfate from the hydroxide.

The alumina trihydrate may be removed from the solution in another way.That is, alumina trihydrate may be precipitated between the range of11.8 to 125 by slowly adjusting the pH of the solution with acid such assulfuric acid down to pH 11.8 and thereafter allowing the pH to adjustupwardly. This procedure is repeated until the pH will not rise abovethe pH of 11.8. This precipitates the crystal form of alumina trihydrateinstead of the gel form. This is the preferred method for recoveringalumina trihydrate.

The slurry 132 is then filtered and rinsed at 134, and the aluminatrihydrate solids 136 are polished at 138 and then redirected as thealumina trihydrate stream 54 to the reaction tank 48 to form aluminumfluoride as previously discussed. The sodium sulfate containing liquidstream 140 from the filter 134 is directed to a second alcoholseparation tank 142 wherein alcohol 144, as noted earlier, eithermethanol or ethanol, is mixed with the liquid stream in a volume ratioof approximately 4:1 alcohol:liquid stream to precipitate sodiumsulfate. The ratio of alcohol to liquid stream can range from 10:1 to1:5, for example. The precipitated sodium sulfate is filtered at 146 andis then directed to a dryer 148 and then storage 150, wherein theresultant sodium sulfate is approximately 99.0% pure. The liquid portion152 is directed from the filter 146 to an alcohol recovery still 154wherein alcohol is separated and directed via stream 156 back to storageunit 144 for reuse in the process, while the water stream 158 isdirected to water recycle storage unit 160 for reuse within the process10, such as at 114.

Alternatively, as shown in FIG. 5, liquid fraction 98 resulting from thefirst and second rinse housings 62 and 66, respectively, having beenfiltered at 63 is directed to tank 116 where the pH is adjusted. Asnoted, the pH is adjusted to form soluble sodium aluminate and insolubleimpurities, e.g., calcium hydroxide, iron hydroxide and magnesiumhydroxide. The insoluble impurities are filtered and directed to tank204 where the pH of the liquid in tank 204 is adjusted. In tank 204, thepH is lowered and the tank heated to precipitate calcium compounds,e.g., calcium sulfate. Typically, the pH is adjusted to a pH less than 1by the addition of an acid such as H₂SO₄. Also, typically the tank isheated to a temperature in the range of 80° to 110° C. The calciumcompounds, e.g., calcium sulfate, are filtered at 205 and then stored instorage tank 206.

Liquid from filter 205 is directed to tank 207 where the pH is adjustedto precipitate iron compounds such as iron hydroxide. Typically, the pHis adjusted upwardly to a pH in the range of 4.5 to 5.5. The precipitateis filtered at 208 and stored in storage tank 209.

Liquid from filter 208 is directed to tank 210 where the pH is againadjusted to precipitate magnesium compounds such as magnesium hydroxide.The magnesium compounds are precipitated by adjusting the pH to a pH inthe range of 10.5 to 12. Thereafter, the magnesium precipitate isremoved at filter 211 and stored in tank 212. Then, liquid stream 213from filter 211 is directed to tank 116 to enhance aluminum recovery.The three compounds recovered, e.g., calcium sulfate, iron hydroxide andmagnesium hydroxide, are relatively pure and thus have good commercialvalue.

As the result of the above process 10, spent potliner material isreduced and recycled into commercially useful ingredients, that is,aluminum fluoride; mullite raw brick material; Abrick material Al₂O₃, Cand SiO₂ useful in cement or glass manufacture. Sodium sulfate, calciumsulfate, magnesium hydroxide and iron hydroxide are also recovered.

EXAMPLE

Sixty tons per day of SPL feed, including caked materials and sweepings,is continuously introduced to the crusher 14 and is processed throughthe steps of the process 10 as described above. Utilizing this process,the 60 tons/day SPL input 12 yields approximately 13 tons/day aluminumfluoride end product, approximately 10 tons/day of a refractorymaterial, and approximately 50 tons/day of reusable salts, e.g., sodiumsulfate, for a total of about 73 tons of recycled solid materials, withthe balance of the starting materials being converted to harmless gasesand salts. In processing this 60 tons/day of SPL input 12, substantiallyall of the cyanides contained therein are destroyed, and substantiallyall of the fluorides are converted to aluminum fluoride as a useful endproduct. Thus, these highly environmentally damaging materials areeither eliminated or converted to useful products.

As can be seen from the above, the present invention provides a highlyefficient process for not only treating the significantly hazardousspent potliner material from aluminum reduction cells, but also servesto convert the components of the SPL to useful end products. Moreover,there are no significant amounts of solid waste material from theprocess of the present invention which must be subsequently disposed ofin landfills or stored, as previously required in other processes andpractices for treating spent potliner material. In addition, the processof the present invention efficiently recycles water and heat andproduces refractory material which can be used in the fabrication of newaluminum reduction cells, thereby providing a highly efficient andeconomic process without a liquid or noxious gas waste stream. Theprimary end products of aluminum fluoride, refractory material andsodium sulfate are all usable, either in the actual manufacture ofaluminum reduction cells or in other commercial endeavors. The storedimpurities of calcium sulfate, iron hydroxide, magnesium hydroxide,ammonium sulfate and calcium chloride are all benign, and are alltreatable in accordance with conventional processes and may be reclaimedfor a wide variety of commercial uses since they include noenvironmentally hazardous materials, such as for water treatment torecover fluoride and solids. As a result, it is seen that the presentinvention is a highly efficient process and very economical in both itsoperation as well as its yield, and that it avoids having to depositfused solid material containing environmentally hazardous component intolandfills or storage.

The foregoing exemplary descriptions and the illustrative preferredembodiments of the present invention have been explained in the drawingsand described in detail, with varying modifications and alternativeembodiments being taught. While the invention has been so shown,described and illustrated, it should be understood by those skilled inthe art that equivalent changes in form and detail may be made thereinwithout departing from the true spirit and scope of the invention. Itshould be further understood that the scope of the present invention isto be limited only to the claims except as precluded by the prior art.Moreover, the invention as disclosed herein may be suitably practice inthe absence of the specific elements or steps which are disclosedherein.

What is claimed is:
 1. A method of recovering AlF₃ from spent potliner material from aluminum reduction cells, which spent potliner material includes at least one material selected from the group consisting of fluoride compositions, cyanide compositions, iron compositions, calcium compositions, magnesium compositions, alumina, carbon, silica and sodium sulfate, comprising the steps of: (a) contacting the spent potliner with an acid in an acid digester to produce a gas component containing HF gas and HCN gas and a slurry component containing at least one component selected from the group consisting of carbon, silica, alumina, sodium sulfate, iron sulfate, calcium sulfate and magnesium sulfate; (b) separating said gas component from said slurry component; (c) after separating, heating said gas component to a temperature in the range of 750° to 850° C. in an oxidizing atmosphere to convert said HCN to CO₂, H₂O and oxides of nitrogen to provide a gaseous mixture containing unreacted HF gas; (d) treating said gaseous mixture with water to form hydrofluoric acid; (e) reacting said hydrofluoric acid with alumina trihydrate to form AlF₃ in an aqueous solution; and (f) recovering said AlF₃ from said aqueous solution.
 2. The method in accordance with claim 1 wherein said acid is selected from the group consisting of H₂SO₄, HNO₃, HClO₄, HCl and H₃(PO₄).
 3. The method in accordance with claim 1 wherein said acid is H₂SO₄.
 4. The method in accordance with claim 1 including maintaining said digester at less than atmospheric pressure to facilitate removing said gas component therefrom.
 5. The method in accordance with claim 1 including maintaining said slurry in said digester at a temperature in the range of 135° to 300° C.
 6. The method in accordance with claim 1 including maintaining said slurry in said digester at a temperature in the range of 135° to 160° C.
 7. The method in accordance with claim 3 including maintaining said sulfuric acid in said digester in a ratio of 0.8 to 1.1 acid to spent potliner on a weight basis.
 8. The method in accordance with claim 1 including reacting said hydrofluoric acid with alumina trihydrate to form said AlF₃ at a temperature range of 100° to 300°.
 9. The method in accordance with claim 1 including drying said AlF₃ to less than 10 wt. % moisture.
 10. The method in accordance with claim 1 including flash heating said AlF₃ to remove water therefrom.
 11. A method of recovering AlF₃ from spent potliner material from aluminum reduction cells, which spent potliner material includes at least one material selected from the group consisting of fluoride compositions, cyanide compositions, iron compositions, calcium compositions, magnesium compositions, alumina, carbon, silica and sodium sulfate, comprising the steps of: (a) contacting the spent potliner with sulfuric acid in an acid digester to produce a gas component containing HF gas and HCN gas and a slurry component containing at least one component selected from the group consisting of carbon, silica, alumina, sodium sulfate, iron sulfate, calcium sulfate and magnesium sulfate; (b) maintaining said slurry component at a temperature range of 135° to 300° C. in said digester; (c) maintaining said digester at less than atmospheric pressure and separating said gas component from said slurry component; (d) heating said gas component to a temperature in the range of 750° to 850° C. in an oxidizing atmosphere to convert said HCN to CO₂, H₂O and oxides of nitrogen to provide a gaseous mixture containing unreacted HF gas; (e) treating said gaseous mixture with water to form hydrofluoric acid; (f) reacting said hydrofluoric acid with aluminum hydroxide to form AlF₃ in an aqueous solution; and (g) recovering said AlF₃ from said aqueous solution. 